A 4 g weighed portion of calcium was dissolved in 1 L of water. Calculate the mass fraction of calcium

A 4 g weighed portion of calcium was dissolved in 1 L of water. Calculate the mass fraction of calcium hydroxide in the resulting solution.

Given:
m (Ca) = 4 g
V (H2O) = 1 L = 1000 ml

To find:
ω (Ca (OH) 2) -?

1) Ca + 2H2O => Ca (OH) 2 + H2 ↑;
2) n (Ca) = m / M = 4/40 = 0.1 mol;
3) n (Ca (OH) 2) = n (Ca) = 0.1 mol;
4) m (Ca (OH) 2) = n * M = 0.1 * 74 = 7.4 g;
5) m (H2O) = ρ (H2O) * V (H2O) = 1 * 1000 = 1000 g;
6) n (H2) = n (Ca) = 0.1 mol;
7) m (H2) = n * M = 0.1 * 2 = 0.2 g;
8) m solution (Ca (OH) 2) = m (Ca) + m (H2O) – m (H2) = 4 + 1000 – 0.2 = 1003.8 g;
9) ω (Ca (OH) 2) = m * 100% / m solution = 7.4 * 100% / 1003.8 = 0.7%.

Answer: The mass fraction of Ca (OH) 2 is 0.7%.